1. Field of the Invention
This invention relates to apparatus and methods for electroplating of metals and, more particularly, to an improved apparatus and method utilizing a double electrolytic cell arrangement for obtaining improved plating uniformity.
2. Description of the Related Art
The process by which electric current is passed through a substance to effect a chemical change is known as electrolysis. The chemical change is one in which the substance loses or gains an electron (oxidation or reduction). The process is carried out in an electrolytic cell, an apparatus consisting of positive and negative electrodes held apart and immersed in a solution containing positively and negatively charged ions. The substance to be transformed may form the electrode, may constitute the solution, or may be dissolved in the solution. Positively charged components of the solution travel to the negatively charged electrode (cathode), combine with electrons, and are transformed into neutral elements or molecules, becoming deposited as a plated layer on the basis metal (substrate). Negatively charged components of the solution travel to the other electrode (anode), give up their electrons, and are transformed into neutral elements or molecules, generally remaining in the solution. When the substance to be transformed is the electrode, the reaction is generally one in which the electrode dissolves into solution as it loses electrons to the external circuit. Electrolysis is used extensively in the deposition of metals from solution (electroplating).
In electroplating, a metal may be transferred to normally conductive surfaces (metals) or to non-conductive surfaces (plastics, glasses, etc.) after the latter have been made conductive by such processes as coating with graphite, conductive lacquer, electroless plating, or coating with metal vapor.
Although some metal coating procedures go back to ancient times, modern electroplating began in 1880 with Alessandro Volta's discovery of the voltaic pile, or battery, which made significant quantities of direct current electricity available. Soon thereafter the battery was employed in depositing lead, copper, and silver. A nodule of copper which had been deposited on a silver cathode could not be removed. In the same year, zinc, copper, and silver were deposited on themselves and on a variety of basis metals (the metals on which the plating is applied), such as gold and iron.
FIG. 1 illustrates a typical plating tank 2 containing copper sulfate (CuSO.sub.4) solution 3. A dc source 4 supplies electric current which is controlled by a rheostat 5. When switch 6 is closed, the cathode bar 7, which holds the work 8 to be plated, is charged negatively. Electrons from the cathode bar 7 transfer to the positively charged copper ions (Cu.sup.++), converting them to atoms of copper metal. These copper atoms become part of the cathode surface and constitute copper plating.
As shown in FIG. 1, the same number of sulfate ions (SO.sub.4 --) are discharged on the copper anodes, thereby completing the electrical circuit. The sulfate ions form a new quantity of copper sulfate, taking copper from the anodes 9 that dissolves in the solution and restores it to its original composition. This procedure is typical of nearly all ordinary electroplating processes--the current deposits a given amount of metal on the cathode and the anode dissolves to the same extent, maintaining the solution uniformity more or less. If the balance is perfect and there are no side reactions or losses, an efficiency of 100% could possibly be realized at both the cathode and the anode.
It is possible to plate a wide variety of alloys of metals by mixing suitable solutions of different metals. In this way, plated brass can be made more or less indistinguishable from cast brass. It is also possible to deposit alloys or compounds of metals that cannot be produced by melting and casting them together. For example, tin-nickel alloy plate has been used commercially for its hardness and corrosion resistance, both of which are superior to that of either metal alone. Other common alloy plates include bronze, gold with varying properties such as different colors or hardnesses, and magnetic alloy plates of such metals as iron, cobalt, and nickel. The latter alloy plates are used for memory drums in computers. Solder plate (Sn--Pb) is used in printed circuit work.
While non-metallic materials have been plated since the mid 1800s, a period of burgeoning growth in the use of electroplated plastic began with the introduction of suitable plastics such as acrylonitrile-butadiene-styrene. The utilization of plated plastics has grown enormously in recent years. The plastic part is first chemically etched and is then sensitized and activated. It is then coated with electroless copper or nickel before further plating. Although not comparable to the adhesion of metals to metals, a useful degree of adhesion is obtained in electroplating plastics.
There are innumerable applications for electroplating and electroforming in manufacturing. Uses of electroplating range from the silver plating of tableware and electrical contacts to the plating of zinc on steel articles to prevent their corrosion. Electroplating and electroforming are widely used for many purposes in the aerospace industry, especially in the fabrication of microcircuitry and small metal parts.
Obtaining a satisfactory result in electroplating requires the control of a variety of parameters. Particularly important among these are current density and distribution, temperature of the solution, diffusion velocity of the metallic ions, shape and structure of the electrodes, and the degree of agitation of the solution. Impurities in the electrolyte solution may also be adsorbed or deposited with the desired metal and may affect the ultimate properties of the plated metal. Because ion deposition does not occur uniformly, considerable attention has been devoted to developing apparatus and methods which allow the control of the various parameters that affect deposition.
Two factors which affect the distribution of current in electroplating are the location of the anode in the plating tank relative to the cathode, and the availability of fresh electrolyte solution at the cathode. It has long been recognized that for better uniformity a plating cell containing multiple anodes interspersed with cathodes can be employed. More uniform current density is achieved in this way, resulting in more uniform plating. Another method for accomplishing accurate deposition makes use of special rings comprising the anode which are fixed in position about the object to be plated.
The importance of anode position in electroplating is shown, for example, in U.S. Pat. No. 3,954,569. This patent discloses anode shields in a process for electroplating tubes or holes in printed circuit boards. By shielding the anode, the effective anode current density can be regulated to allow for uniform plating at the cathode.
The role of electrolyte concentration in the plating process is demonstrated by the buildup of metal deposition at the outer edges of a plated object. Efforts to control this phenomenon involve using "robbers" or "thieves," which are strips of metal situated around the edges of the object. The "robbers" or "thieves" are plated to reduce the excess plating of the edges. This process is expensive because of the extra metal required and additionally because extra power is consumed in the process.
Another method for minimizing the effect of electrolyte concentration at the cathode is to shield the object being plated by surrounding it with a non-conductive inert material. This essentially forms a cathode shield around the object and reduces the buildup of current density at the outer edges. The regulation of current density at the cathode is of key importance in effecting satisfactory plating. It is of crucial importance when objects having fine detail are being plated.